FIG. 1 shows a simple motor system, indicated generally by the reference numeral 1. The motor system 1 comprises a three-phase motor 2, an AC power source 4, a rectifier 6, an inverter 8 and a control module 10.
The output of the AC power source 4 is connected to the input of the rectifier 6. The output of the rectifier 6 provides DC power to the inverter 8. In a manner well known in the art, the inverter module includes a switching module, typically comprising insulated gate bipolar transistors (IGBTs) that are driven by gate control signals in order to convert the DC voltage into an AC voltage having a frequency and phase dependent on the gate control signals. The gate control signals are provided by the control module 10. In this way, the frequency and phase of each input to the motor 2 can be readily controlled.
The inverter 8 is in two-way communication with the control module 10. The inverter typically monitors currents and voltages in each of the three connections to the motor 2 and provides that current and voltage data to the control module 10 (although the use of both current and voltage sensors is by no means essential). The control module 10 may make use of the current and/or voltage data (where available) when generating the gate control signals required to operate the motor as desired; another arrangement is to estimate the currents from the drawn voltages and the switching pattern—other control arrangement also exist.
The motor 2 may be used in a wide variety of applications. In some applications, it may be important that the motor 2 functions, even if the AC power supply 4 fails. For example, the motor 2 may be used to operate a cooling fan. If the motor 2 fails, then the cooling fan does not operate and, unless other control arrangements are provided, the device being cooled may overheat.
In such circumstances, it is known to provide a backup power supply, in the event that the AC power supply 4 fails. For example, a bank of batteries providing a DC power source may be provided in the event that the AC power source 4 fails.
Although the use of AC and DC power supplies for a motor drive system is known, there remains a need for improved algorithms for controlling such systems. In particular there remains a need for novel algorithms for controlling the entry into an emergency mode of operation in the event that the AC power supply fails and the exiting of the emergency mode of operation in the event that the AC power supply is restored.
The present invention seeks to address at least some of the problems outlined above.